Concrete forms of various types have long been widely used in construction work. When the structure that is to be built has straight sides, the forms are usually constructed from rigid steel or wood. If curved shapes are required, other materials are normally used, including flexible metal, masonite, lap siding, and strips of thin plywood which can be bent into the desired shape. If a structure has a straight portion that connects with a curved area, different types of forms must be used for the different sections, and there can be problems in the transition area where they meet. When different materials are used for the forms in the straight and curved areas, the transition area is often difficult to set or finish properly.
All of the materials that are commonly used for concrete forms are easily damaged. If steel forms are dented or bent, they are essentially useless. Wood forms are difficult if not impossible to use over and over again. Moreover, wood is a precious resource, and thus undesirable for use as a disposable form material. Also, concrete can stick to the forming faces of wood, steel and other materials to the point where adequate cleaning is impossible. Release from the concrete once it has set usually requires the use of a release agent or labor-intensive scraping which complicates the construction process and add to the costs. Handling of many types of forms is difficult because of their weight and bulk.
U.S. Pat. Nos. 4,579,312 and 4,712,764 to White disclose flexible forms in which rubber is the preferred material. Although the patents disclose use of these forms to provide curved shapes, the forms are lacking in strength and rigidity and are not well suited for straight shapes. Additionally, these forms are designed for flexure of a horizontal nature, but are prohibited from flexure of a vertical nature by their very construction. They are also characterized by complicated anchoring systems. U.S. Pat. No. 5,154,837 to Jones discloses another flexible form. It includes a core which is flexible enough to allow bending but rigid enough to prevent the outer steel structure from kinking. Again, the necessary strength and rigidity is lacking to permit this type of form to be successfully used for the construction of long, straight runs. These forms are also cost-prohibitive to manufacture.